1. Field of the Invention
The present invention relates to a system for current overload protection in DC-DC converters and to the corresponding method.
2. Discussion of the Related Art
As is known, DC-DC converters are electronic components important for proper operation of electronic systems, which, being supplied, for example, by a common generator, require operative voltages that are different from one another. For example, DC-DC converters are used in cellphones, laptops, and in general in battery-supplied electronic systems.
Frequently, the subcircuits used in these electronic systems require operating voltages different from the one supplied by the battery, and typically lower. Consequently, DC-DC converters are commonly used and generate a voltage level different from the converter input voltage.
FIGS. 1a and 1b illustrate, as an example, two possible embodiments of a DC-DC converter 1 of a “buck converter” type without any protection circuit. However, this configuration should not be considered in any way limiting the reference application field, in so far as considerations similar to the following may apply to DC-DC converters of other types, for example flyback converters and boost converters.
With reference to FIGS. 1a, 1b, the DC-DC converter 1 comprises a first and a second switch 2, 3, typically formed by bipolar, N-channel or P-channel MOSFETs, or diodes.
In particular, FIG. 1a shows a DC-DC converter 1 with free-wheeling diode.
The first switch 2, in this example a MOS transistor, has a first terminal, which is connected to an input terminal 4 of the DC-DC converter 1 and receives a d.c. voltage V1, a second terminal connected to a node 5, and a control terminal receiving a voltage signal PWM_HS. The node 5 is connected to a ground terminal 6, through the second switch 3.
The second switch 3, in this example a diode, has a first terminal connected to the node 5 and a second terminal connected to the ground terminal 6. The node 5 is moreover electrically connected to an inductive element 7, a capacitive element 8, and a load 9.
In use, when the first switch 2 is on, it is flown by a current IL, coming from the input terminal 4 and flows therefrom to the node 5 and then through the inductive element 7. In this condition, the diode 3 is reversely biased and does not conduct.
When the first switch 2 is turned off, the voltage across the inductive element 7 is reversed, thus directly biasing the diode 3, which sets the voltage drop on the load to the value of approximately 0 V.
FIG. 1b shows, instead, a DC-DC converter 1 of the buck-converter type with synchronous rectification.
In this case, the second switch 3 is obtained using a MOS transistor. Here, the second switch 3 has a control terminal receiving a control signal PWM_LS.
To ensure robustness of DC-DC converters during use, it is known to interface these converters with protection circuits having the function of preventing breakdown or damage to electrical components coupled at the output of the converter and to load elements, in faulty operating conditions. The systems known and used for protecting DC-DC converters from current overload and/or from short-circuits on the output enable limitation of the current supplied to the load by reducing the duty cycle or decreasing the on/off rate at which the DC-DC converter operates (J. Yang, “Analysis and evaluation of over current protection for DC to DC PWM converters,” Power Electronics and Motion-Control Conference, 2004, and U.S. Pat. No. 6,218,820).
FIG. 2 shows a first circuit 14 for overcurrent protection connected, for example, to the synchronous-rectification DC-DC converter 1 of the type illustrated in FIG. 1b. 
An overcurrent detector 15 receives a signal correlated to the current IL flowing in the induction coil 7 during turning-on of the first switch 2 and outputs a threshold-overstepping signal OCP.
For example, the overcurrent detector 15 detects the voltage across a “sense” resistor (not illustrated), connected in series to the first switch 2, and comprises a threshold comparator, which compares the detected voltage with a reference value.
The threshold-overstepping signal OCP is then supplied to the input of a controller 16, which in turn, on the basis of the threshold-overstepping signal OCP, outputs a duty-cycle signal PWM.
The duty-cycle signal PWM is then supplied to the input a driving circuit 20, which controls opening or closing of the first switch 2 and of the second switch 3, respectively, through a first turning-on signal PWM_HS and a second turning-on signal PWM_LS. When the DC-DC converter 1 is of the free-wheeling diode-type as illustrated in FIG. 1a, the second turning-on signal PWM_LS is not necessary, and only the first turning-on signal PWM_HS is supplied to the first switch 2.
A feedback branch electrically connects the load 9 to an error amplifier 19, which receives on a first input an output signal V0 coming from the load 9 and on a second input a reference signal VREF. An error signal at the output of the error amplifier 19, given by the difference VREF−V0, is supplied to the controller 16.
If the overcurrent detector 15 does not intervene, the difference signal VREF−V0 is kept at approximately 0 V. In fact, when the load 9 varies, the voltage signal V0 undergoes a variation of opposite sign, which is detected by the controller 16. This modifies the on/off time (i.e., the duty cycle) of the first switch 2, so as to bring the voltage signal V0 back to the steady-state value.
The overcurrent detector 15 contributes to implementing a first overcurrent protection technique, which is commonly referred to as “peak-limitation technique” and can be used with both the free-wheeling diode configuration (FIG. 1a) and the synchronous rectification configuration (FIG. 1b) of the DC-DC converter 1. In particular, at each on/off cycle (timed by a clock signal), when the current that flows through the first switch 2 exceeds a maximum peak level (of a value that varies according to the tolerance required by the application), the first switch 2 is kept off until the end of the current clock cycle.
The waveforms obtainable with the protection technique described are, for example, represented in FIG. 3, which shows a clock signal CLK, the first turning-on signal PWM_HS and the current IL that flows through the inductive element 7.
In detail, at each transition from a low level to a high level of the clock signal CLK, the first turning-on signal PWM_HS also switches from low to high, turning on the first switch 2, which, in this step, supplies the current IL. Consequently, the value of the current IL increases. In the presence of a possible overcurrent, the current IL reaches a protection threshold Pk. Exceeding the protection threshold Pk is detected by the overcurrent detector 15, which switches and causes, through the controller 16 and the driving circuit 20, switching of the first turning-on signal PWM_HS to low. The first switch 2 is turned off and the second switch 3 is turned on, so causing a reduction in the current IL.
Turning-on of the second switch 3 can be controlled by the signal PWM_LS if the synchronous rectification configuration of the DC-DC converter 1 is used (FIG. 1b), or else occurs automatically after turning off the first switch 2 if the free-wheeling diode configuration of converter (FIG. 1a) is used.
The peak protection technique illustrated above is not, however, sufficient to guarantee robustness of the DC-DC converter 1, in so far as, in the event of marked current overload, the average value of the current on the output of the DC-DC converter 1, and thus on the load 9, can be considerably high notwithstanding the use of the first protection circuit 14.
To overcome this problem, it is possible to implement a second overcurrent protection technique, referred to as “hiccup.” The hiccup protection technique can be used in addition to the peak protection technique, and envisages the use of a protection threshold PH (not illustrated) higher than the protection threshold Pk. When the current at the output of the DC-DC converter 1 exceeds this protection threshold PH, the intervention of the hiccup protection causes complete switching-off of the DC-DC converter 1.
A third overcurrent protection technique, which can be used as an alternative to the peak protection technique, is referred to as “trough-limitation technique.” As illustrated in FIG. 4, this technique envisages the use of a protection threshold Vy. Also in this case, at each transition from low to high of the clock signal CLK—21, the first turning-on signal PWM_HS switches from low to high, so turning on the first switch 2 and generating a consequent increase in the value of the current IL. After a fixed time, defined by the duty cycle chosen for the first turning-on signal PWM_HS, the latter switches from high to low, and the second turning-on signal PWM_LS passes from low to high for a fixed duration, defined by the duty cycle chosen for the second turning-on signal PWM_LS. Consequently, the value of the current IL starts decreasing. If, at the start of the subsequent clock cycle, the current IL has a lower value than the value Vy of the protection threshold, then the first turning-on signal PWM_HS switches again from low to high. If, instead, at the start of this cycle, the current IL is higher than the protection threshold Vy, the first turning-on signal PWM_HS remains low, and the second turning-on signal PWM_LS remains high for the entire duration of the considered clock cycle.
The hiccup protection technique can be associated also to the trough-limitation technique, as further protection of the DC-DC converter 1.
To sum up, the peak-limitation technique reduces the duty cycle of the first turning-on signal PWM_HS and maintains fixed the on/off rate of the first and second switches 2, 3; the trough-limitation technique maintains fixed the duty cycle of the first turning-on signal PWM_HS and decreases the on/off rate of the first and second switches 2, 3.
It is evident that the trough-limitation technique cannot be used with DC-DC converters in free-wheeling diode configuration (FIG. 1a).
Furthermore, the output voltage V0 is out of regulation during opening of the switch 2.
Consequently, when the trough-limitation technique is used, it may happen that the switch 2 remains open on account of intervention of the protection and that this causes decrease in the output voltage.
When the current that flows through the second switch 3 drops below the trough value, the first switch 2 is closed (after opening of the second switch 3) and this could remain closed for an entire clock cycle (100% of the duty cycle) in order to try to recover the decrease in the output voltage. Since no control is present on the current that traverses the first switch 2, this could increase excessively during this time generating malfunctioning on the load 9, even up to breakdown of the latter.
Neither the peak-limitation technique nor the trough-limitation technique, if used individually, is consequently able to guarantee a sufficient robustness of the converter to which it is applied.
The simultaneous use of the two techniques described and the provision of the circuitry necessary for their implementation would introduce a high circuit complexity, linked to the need for two distinct protection circuits 14 operating on the first and second switches 2, 3. This solution is consequently markedly disadvantageous and complex to implement.